Stretchability is not something you'd think of as synonymous with electronics. For this very reason the realm of wearable electronic devices has been limited to devices on clothes with rigid or at best semi-flexible circuit boards or solar panels and watches that can do just about everything except make a decent espresso. The game is about to change with the introduction of a silicon nanowire with elastic properties that could enable the incorporation of stretchable electronic devices into clothing, implantable health-monitoring devices, and a host of other applications.
Researchers at North Carolina State University (NCSU) have created the first silicon nanowire coils that can be stretched to more than double their original length. Initial experimenting had been with folding electronic materials, which can stretch much like the bellows of an accordion, however this design was found to be flawed. Failures in the peaks and valleys of the waves were causing the entire structures to fail. Dr Yong Zhu, one of the researchers at NCSU then came across the idea of creating coiled structures to eliminate this problem.
“In order to create stretchable electronics, you need to put electronics on a stretchable substrate, but electronic materials themselves tend to be rigid and fragile,” Dr Zhu said. “Our idea was to create electronic materials that can be tailored into coils to improve their stretchability without harming the electric functionality of the materials. An ideal shape to accommodate large deformation would lead to a uniform strain distribution along the entire length of the structure – a coil spring is one such ideal shape. As a result, the wavy materials cannot come close to the coils’ degree of stretchability.”
To create the coiled nanowire Zhu’s team put a rubber substrate under strain and used very specific levels of ultraviolet radiation and ozone to change its mechanical properties. They then placed silicon nanowires on top of the substrate, which formed coils upon release of the strain. Other researchers have been able to create coils using freestanding nanowires, but have so far have been unable to directly integrate those coils into a stretchable substrate.
Currently the team are working to optimize the electric performance of the coils which can be stretched to 104 percent beyond their original length. At this full extension the materials lose some of their electrical efficiency due to factors of contact resistance change and electrode failure. The researchers are hopeful they can overcome these problems.
“We are working to improve the reliability of the electrical performance when the coils are stretched to the limit of their mechanical stretchability, which is likely well beyond 100 percent, according to our analysis,” Dr Zhu said.
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